1. Field of the Invention
The invention relates to an electrochromic cell for a rearview mirror. More particularly, the invention relates to an electrochromic cell for a rearview mirror including a stack of layers consisting of a layer of inorganic and transparent electrochromic material.
2. Description of the Related Art
An electrochromic cell typically includes layers of an inorganic and transparent electrolyte designed to establish a potential difference between these layers and the electrochromic material, and the electrolyte being chosen so that under the influence of this potential difference, cations migrate in a reversible way from the electrolyte towards the electrochromic material in order to be inserted into this last causing a modification of its power of absorption of the light.
It also concerns applications of this cell and a realization method of this one. In the known electrochromic cells of above-mentioned type, use is generally made, as electrochromic materials, of transition metal compounds, particularly, the tungsten oxide (W03) and vanadium oxide (V2O5).
These last two materials are indeed likely to integrate, by insertion, cations such as for example (Fr), Li+, Na+ and K+ protons.
This insertion is accompanied by a chemical modification of materials, at the origin of a change of color, that is to say of an increase in the capacity of absorption of the light.
Thus, for example, WO3 is colorless in a normal state and, due to the insertion of cations, is transformed into a colored complex with blue color.
It should be added that the insertion of cations is obtained by choosing the above-mentioned electrolyte so that this one, under the influence of a potential difference between this electrolyte and the electrochromic material, liberates cations, which come to be inserted in the electrochromic material.
It should be specified that this phenomenon of insertion is reversible, in the sense that by applying a potential difference inverse to the one causing the insertion of cations, there is migration of these same cations in return towards the electrolyte, which is accompanied by the transformation of the above-mentioned colored complex into colorless W03.
In the known cells of above-mentioned type, the electrolyte is constituted of a solid, organic or inorganic ionic conductor.
However, stacks consisting of an organic ionic conductor, such as polymer, are difficult to realize with a high degree of precision and uniformity of thickness because of the liquid or pasty characteristics of this type of conductor, characteristics which are given to it due to the presence of solvents; this lack of precision and uniformity is at the origin of the formation of marbling during the polarization of the stack.
Furthermore, the stage of realization of the layer of organic ionic conductor (injection of the conductor in the space formed between two glass layers with constant distance, then sealing this space) is not only difficult to implement, but also does not become integrated with the realization method of other layers of the stack, method which is a type of physical vapor deposition; it follows that two different manufacturing techniques are necessary to realize the stack, which complicates the manufacturing of this last.
Moreover, in the case of inorganic ionic conductors such as hydrated Ta2O5: H, ionic conduction varies in an extremely fast way with the degree of hydration; it is consequently essential to watch out that the preparation conditions of such hydrated conductor allow to acquire, in a precisely and reliably way, the desired degree of hydration, which is extremely difficult in the industry scale.